Eubranchus flexus, Forbes, 1838

Grishina, Daria Yu., Antokhina, Tatiana I. & Ekimova, Irina A., 2023, A new species of the genus Eubranchus (Gastropoda: Nudibranchia) from Vietnamese coastal waters, Ruthenica, Russian Malacological Journal 33 (1), pp. 1-8 : 4-5

Eubranchus flexus sp. nov.

( Figs 2–4 )

ZooBank registration: urn:lsid:zoobank. o r g: a c t: 8 F 3 D 3 9 2 E - E F C 3 - 4 8 C 7 - A 0 4 E - 89D41C125453

Type material. Holotype ZMMU WS 19111 , Vietnam, Nha Trang , Hon Tre Island, Dam Bay, 12°11.688’N. 109°17.386’E, 3 m in depths, 27.10.2016, coll. – T.Antokhina. One paratype (from the type locality): ZMMU WS 19112 .

Type locality. Vietnam, Nha Trang , Hon Tre Island, Dam Bay, 3 m depth, 12°11.688’N, 109°17.386’E .

Description. External morphology ( Fig. 2 A, B ). Length 6–17 mm. Body elongate, narrow. Foot narrow, significantly wider anteriorly. Head large, bearing oral tentacles and rhinophores. Rhinophores elongated, smooth. Oral tentacles elongated, smooth. Rhinophores of same length as oral tentacles. Cerata arranged in 5–6 groups with 1–3 cerata per group. Cerata located opposite to each other. Cerata smooth, winging, curved, pointed distally. Reproductive opening located laterally under first ceratal row at right. Anal opening acleioproctic.

Coloration ( Fig. 2A, B ). Background color translucent whitish. Rhinophores, oral tentacles, head, dorsal and dorsolateral sides of body, except areas between groups of cerata, covered with densely arranged reddish-brown small, rounded spots. Midline of body and areas with no brownish spots with small clusters of opaque white and yellowish pigment. Cerata bases transparent. Clusters of white and yellowish opaque spots of irregular shape located along distal parts of foot on each side. Cerata semitransparent, milky-white, covered with large semitransparent brownish spots of irregular shape (up to 2/3 of cerata length), and with ramdomly scattered opaque white and yellowish pigment. Cnidosac area with opaque white pigment on ceratal tip and subapical reddish-brown pigmental ring. Rhinophores and oral tentacles semitransparent, with intense opaque-white pigmental aggregations of irregular shape and yellowish apical ring and brownish ring in middle part.

Internal morphology ( Figs 3, 4). Paired jaws elongate, thin, delicate, masticatory border of jaw with small denticles. Radula triserial, radular formula 58 × 1.1.1. Rachidian tooth with 3 denticles on each side. Central cusp separated from lateral denticles by small gap. Lateral teeth wide, 2–3 times wider than rachidian tooth, with rectangular base and triangular cusp placed close to rachidian tooth. Lateral teeth with small triangular denticle on outer side of cusp. Reproductive system diaulic. Receptaculum seminis spheric. Vagina wide. Ampulla elongated, narrow, tubular. Vas deferens short and wide. Penial gland present, elongated, with short duct.

Distribution. South China Sea. This species is known only from the type locality.

Ecology. ( Fig. 2C ). Specimens were found on hydrozoan colonies of the family Sertulariidae with their egg masses. The egg mass is deposited on hydrozoan as an irregular loop, beige in color.

Remarks. Both morphological and molecular analyses support the distinctiveness of Eubranchus flexus sp. nov. from other species in the genus ( Fig. 1). Morphologically, there is no other described species of the genus Eubranchus , demonstrating similar coloration, external appearance and radular characters (see Discussion for details). Eubranchus flexus sp. nov. is a single representative of the genus Eubranchus that described from Vietnam.

Etymology. From Latin “ flexus ” (bending, winding, curvilinear), referring to specific shape of cerata in this species.

Figures

FIG. 2. Living specimens of Eubranchus flexus sp. nov. A. Holotype ZMMU WS19111, dorsal view. B. Paratype ZMMU WS19112, dorsal view, specimens was damaged during collection. Size of the fixed specimens is around 4 mm. C. Eubranchus flexus sp. nov. specimens with egg mass and the host hydrozoan colony; white arrows point to egg masses, black arrows with a white outline point to specimens. РИС. 2. ПриЖиЗненные фотографии Eubranchus flexus sp. nov. A. Голотип ZMMU WS19111 вид с дорсальной стороны. B. Паратип ZMMU WS19112. РаЗмер Зафиксированных обраЗЦов составлЯет около 4 мм. C. Особи Eubranchus flexus sp. nov. на гидроиде; белые стрелки укаЗывают на кладки ЯиЦ, черные стрелки с белым контуром укаЗывают на особей моллюсков.